The present invention relates to a novel electrolysis of an aqueous solution of sodium chloride for producing chlorine in an anolyte compartment and sodium hydroxide in a catholyte compartment in high current efficiency.
A diaphragmic electrolysis of an aqueous solution of sodium chloride in two compartments has been known.
In said electrolysis, an anode and a cathode in an electrolytic cell are partitioned with a diaphragm and an aqueous solution of sodium chloride is fed into the anolyte compartment to electrolyze it to produce chlorine in the anolyte compartment and to produce sodium hydroxide in the catholyte compartment.
Heretofore, asbestos has been usually used as a diaphragm. When asbestos diaphragm is used as a diaphragm, sodium chloride is contaminated in the resulting sodium hydroxide whereby the purity is decreased disadvantageously. Moreover, asbestos itself is corroded by a base in high concentration whereby a solution of sodium hydroxide in high concentration can not be obtained.
Accordingly, it has been proposed to use a fluorinated cation exchange resin having a chlorine resistance and a selective permeability for sodium ions as a diaphragm.
When the fluorinated cation exchange membrane is used as a diaphragm, the contamination of sodium chloride in the resulting sodium hydroxide can be prevented because of the ion selectivity thereof.
The methods have been comparatively satisfactory from the viewpoint of purity, however, the current efficiencies have not been satisfactory. According to the experiments by the inventors, the practical current efficiencies in the cases of using the conventional cation exchange membranes have been up to about 80 percent as disclosed in the publications such as U.S. Pat. specification No. 3,773,634. When the concentration of sodium hydroxide has been high, the current efficiencies have been remarkably low.
The unsatisfactory current efficiencies have been caused by a leakage of hydroxyl ions formed in the catholyte compartment through the cation exchange membrane to the anolyte compartment in the condition of electrolysis. However, it has not been known to obtain a membrane and a method which are effective for inhibiting the leakage of OH.sup.- through the cation exchange membrane.
The inventors have studied to produce sodium hydroxide in high current efficiency by the electrolysis of an aqueous solution of sodium chloride by using a cation exchange membrane as the diaphragm.
As the result, the inventors have found novel fact that sodium hydroxide can be produced in unpredictably high current efficiency by controlling a water permeability of the cation exchange membrane in desired range in the electrolysis when the fluorinated cation exchange membrane having carboxylic acid groups as ion-exchange groups is used as the diaphragm.